1. Field of the Invention
The present invention relates to a switching power source circuit and more particularly to a switching power source which is useful when an output voltage to be obtained on a secondary side in a current resonance type switching power source is used as a synchronous rectifying method.
2. Description of Related Art
Recently, in association with saving energy in the global environment, various switching power sources have been requiring higher efficiency with noise reduction.
Particularly, as for a power source circuit for such as a computer, communication equipment and the like, a dc--dc converter where high efficiency is maintained and noises are small even when a low voltage output, has been requested.
However, generally, if a low voltage is outputted, in case of having the same consumption power, an output current becomes a large current. In case of the dc--dc converter, a resistance loss due to a rectifying diode on a secondary side indicates a large power loss.
Therefore, it is considered that by using a current resonance type switching power source where noises are relatively small with high efficiency, and a rectifying device to output by a low on-resistor on the secondary side, for instance, an MOS transistor, a DC output voltage is derived by rectifying by a synchronous rectifying method.
FIG. 4 shows an example of a switching power source circuit using such a combination. Reference symbols Q1 and Q2 denote switching elements which comprise MOSFET, serially connected, respectively. A reference symbol T denotes an isolation transformer for transferring a switching power on a primary side to a secondary side.
IC denotes a signal source for alternately turn on/off the switching elements Q1 and Q2 at a predetermined switching period and, regularly, is constructed so as to enable a switching frequency of the switching elements to be varied while comparing an output voltage V0 with a reference voltage by voltage detecting means (not shown) for controlling so that the output voltage V0 can be set to a constant voltage.
The output of the switching elements Q1 and Q2 is supplied to a primary winding L1 of the isolation transformer T and a resonance capacitor C1. If the switching elements Q1 and Q2 are alternately turned on/off, the primary winding L1 of the transformer is driven by a current to charge/discharge the resonance capacitor C1 to resonate to a leakage inductance of the transformer T. As shown in FIG. 5, a voltage V1 which is applied to the primary winding L1 is induced as V2 to a secondary winding L2. In case of the normal dc--dc converter, by a pair of diodes for rectifying, a full wave rectification is performed.
However, if the output voltage is low, loss by the rectifying diodes is remarkably large. As shown in FIG. 4, thus, in place of the rectifying diode, by using N-channel MOS transistors Q3 and Q4, a full wave rectification is performed by a synchronizing method. A circuit where the DC voltage V0 is outputted from a smoothing capacitor C0 is constructed.
In case of the circuit in FIG. 4, the smoothing capacitor C0 is charged with a full wave rectifying voltage via the MOS transistors Q3 and Q4 under low resistance.
"D" shows a parasitic diode which is made up by the MOS transistors Q3 and MOS transistors Q4.
At the time of turn-on, a current resonance type switching power source in which the switching elements are half-bridge-connected is set so as to execute a zero current switching. Since the power source is set to resonate to a current at the turn-off time, the power source is characterized in that the noises are essentially small and the output voltage V0 on the secondary side can be widely varied by changing the switching frequency. However, in order to assure a wide regulating range, in the whole period, there is also a case where the power source has a rectifying current continuous mode for transmitting a current to the secondary side, and a secondary side rectifying discontinuous mode for which a current is not supplied on the secondary side.
However, conventionally, the output voltage or the current of the isolation transformer T is detected and a logic circuit to control the MOS transistors Q3 and Q4 is built in. For example, control circuits are provided so that voltages at "a" point and "b" point in an output circuit in FIG. 4 are detected, a proper on/off control signal is generated, and the MOS transistors Q3 and Q4 are conducted.
If such a circuit is, however, provided, the following problems are caused. The number of parts is increased. As shown in FIG. 5, when the rectifying device is turned on/off while detecting the voltage V2 at the output point, a time point C to actually detect the voltage is delayed from the generation time point of the output voltage V2 of the isolation transformer. Since the rectifying device enters in a conductive state from the time point C and a current Id flows, a rectifying current id is delay controlled and the efficiency of the rectifying operation is low.
The control voltage detection delay causes the timing at which both rectifying device Q3 and Q4 are turned off, resulting in the problem where conduction angle of rectifying current is narrower, against which electric power transferring rate should be lowered by lowering the power factor.